The prevalence of obesity has risen dramatically among children in the U.S. since the 1960s. Effective treatment of childhood obesity is widely recognized as instrumental to public health efforts to combat type 2 diabetes and heart disease. Evidence from animal studies, short-term human studies and epidemiology suggests that diets designed to minimize the rise in postprandial blood glucose, that is low glycemic index, may be useful in the treatment of obesity and related complications. This project proposes two studies involving obese adolescent subjects age 13 to 18 years: 1) a randomized controlled trial examining the effectiveness and practicality of a low glycemic index diet in the treatment of obesity and prevention of related complications; 2) a cross-over feeding study investigating the physiological effects of weight-maintaining diets differing in glycemic index on insulin resistance and CVD risk factors under controlled conditions. Study 1 involves 90 subjects randomly assigned to receive standard dietary treatment or a macronutrient-controlled low glycemic index diet. Each subject and at least one parent will received intensive nutritional counseling for 6 months, with follow-up for an additional 6 months. Both diet groups will receive identical treatment intensity, behavioral modification and physical activity recommendations. The primary endpoint will be change in percent body fat by dexa-scan at 12 months; other outcomes include insulin resistance and CVD risk factors. Compliance will be assessed by interviewer-administered, 24-hour dietary recalls. Study 2 involves 24 subjects who will be randomly assigned to received marconutrient-controlled low or high glycemic index diets prepared in a metabolic kitchen for 4 weeks. Subjects will be admitted to the GCRC at baseline and at the end of each dietary treatment for measurement of insulin sensitivity by hyperinsulinemic euglycemic clamp, endothelium-dependent flow-mediated dilation of the brachial artery and other cardiovascular disease-related outcomes. The two studies are expected to provide complementary information regarding the effects of dietary glycemic index on childhood health. The project's key personnel and consultants include individuals with a diversity of expertise and perspective, ensuring a rigorous and impartial evaluation of study hypotheses.